How can I validate that someone is over 18 from their date of birth?

Try this.

var enteredValue = $get('<%=ui_txtDOB.ClientID %>');;
var enteredAge = getAge(enteredValue.value);
if( enteredAge > 18 ) {
    alert("DOB not valid");
    enteredValue.focus();
    return false;
}

Using this function.

function getAge(DOB) {
    var today = new Date();
    var birthDate = new Date(DOB);
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;
    }    
    return age;
}

Demo here: http://jsfiddle.net/codeandcloud/n33RJ/

I think a better alternative would be to calculate the age of the user, and use that in your if statement.

See this SO answer on how to do just that:

Calculate age in JavaScript

<script>


    function dobValidate(birth) {


        var today = new Date();
        var nowyear = today.getFullYear();
        var nowmonth = today.getMonth();
        var nowday = today.getDate();
        var b = document.getElementById('<%=TextBox2.ClientID%>').value;



        var birth = new Date(b);

        var birthyear = birth.getFullYear();
        var birthmonth = birth.getMonth();
        var birthday = birth.getDate();

        var age = nowyear - birthyear;
        var age_month = nowmonth - birthmonth;
        var age_day = nowday - birthday;


        if (age > 100) {
            alert("Age cannot be more than 100 Years.Please enter correct age")
            return false;
        }
        if (age_month < 0 || (age_month == 0 && age_day < 0)) {
            age = parseInt(age) - 1;


        }
        if ((age == 18 && age_month <= 0 && age_day <= 0) || age < 18) {
            alert("Age should be more than 18 years.Please enter a valid Date of Birth");
            return false;
        }
    }



</script>